The present invention relates to a projection type image display device provided with an unauthorized use inhibiting mechanism. These days, uses of projection type image display devices inclusive of projectors are rapidly spreading in a variety of applications, such as presentation at conferences, lecturing in the fields of education, viewing films at home theaters, and so on. Because of the relative easiness of carrying the display devices, though they are expensive products, damages of theft of the display devices are tending to increase. As countermeasures against theft and unauthorized use of them, manufacturers are providing the products with such functions as to nullify changes made to settings and to lock the screen with use of password protection. An example of such art is disclosed in Japanese Patent Unexamined Publication No. 2004-77967.
According to the above mentioned art disclosed in Japanese Patent Unexamined Publication No. 2004-77967, a user at the time when starting up the device is requested to input his password and, only when the input password coincides with the password previously input by the rightful user, operation of the device is allowed and, thereby, unauthorized use of the device is prevented. However, it has been necessary even for a rightful owner of the device, when he uses the device, to input his password for verification of his ownership and therefore operation of the device has been very troublesome.